PC shutting down help?

Alright, I thought I was done with this but it would seem my computer is shutting down when ever I put any real use to it, sense I'm a gamer, that's basically what I have it for, about a month, maybe a month and a half ago, I upgraded my system, replacing the motherboard, Ram, Hardrive and video card, keeping the PSU, CPU and the hardrive as a storage unit,

About 40 minutes to an our ago, or so, my computer shut it self down but booted right back up, the first time, the second time it shut down but I think, I think, I'm not sure, but when it shut it self down the second time it seemed like it tried to stay powered but failed, then would not turn on until now.

I'm thinking its my power supply, I have a corsair 850w but the fact that it is now on makes me wonder if it is, so I'm asking for thoughts on what it might be, every thing in it now is basically new, apart from the CPU and the power supply, oh and the harddrive.

When I tried to turn it on the second time it shut down, nothing turned on, got no responds what so ever, no fans, nothing, I wanna say its the power supply but part of me says it could be the motherboard but its brand new so...

Looking at your system config (based on your signature) it looks like you are WAY over powered for the hardware you have, unless you are running a big bank of SATA drives. You are closer to the 550W to 600W mark. Believe it or not, having too much "extra breathing room" in a PC power supply is not a good thing, as the PSU will have to reconvert all of that extra generated power, which causes heat and component stress. Safely, you want to be within 20% (example, a 300W requirement could safely be met by 390W (90=300*.20), as 20% is essentially the electronically safe limit. You likely are close to 40% surplus, and this often does burn PSUs out quickly.

Yes, did a very thin layer of articsilver 5, watched many videos on youtube before doing so, so I'm pretty sure I did that right, plus I've been watching my temps and I've yet to have one part go past 60c both idle and under load.

Though I don't plan to buy any more hard drives, I do plan to buy a second video card for crossfire, a second 6950, newegg suggests I get a powersupply that's at least 750w if I crossfire, 500w if I only have one.

Okay, this is odd, I just woke up from a mmmm, 3 or 4 hour nap and my computer would not turn on, waited and waited then just as I gave up and told myself that the power supply finally completely gave out . . . With out me hitting the power button again, the computer turned on and seems to be running fine...

So, now I'm pretty sure its the power supply that's dieing but I've never had anything but a video card in my computer die on me before so I couldn't say for sure but

What do you guys think, is it the power supply or could it be something else?